Hardening process for high speed steel tools and other articles



Jan. 3, 1939. w MACHLET 2,142,139

HARDENING PROCESS FOR HIGH SPEED STEEL TOOLS AND OTHER ARTICLES Filed July 18, 1955 3 Sheets-Sheet l www Jan. 3, 1939. MACHLET 2,142,139

HARDENING PROCESS FOR HIGH SPEED STEEL TOOLS AND OTHER ARTICLES Filed July 18, 1935 5 Sheets-Sheet 2 A. W. MACHLET Filed July 18, 1935 3 Sheets-Sheet 3 Jan. 3, 1939.

HARDENING PROCESS FOR HIGH SPEED STEEL TOOLS AND OTHER ARTICLES Patented Jan. 3, 1939 nmnanmo rnocass FOR HIGH SPEED STEEL TOOLS AND o'rrma ARTICLES Adolph W. Machlet, Elizabeth, N. J.

Application July 18, 1935, SerialNo. 31,985 V I 4 Claims. ((21. 148-16) 7 1 I r i quent cooling is also attended with unusual difiiculties.

The tool must be finished before hardening, and must retain its sharp cutting edge and its shape during the severe treatment, and thenmust cut equally well over its entire shape. Its

hardness must be even, so that it will out just 20 aswell at one place as at another. The tool must not be exposed to injury. Moreover it must not suffer more severe treatment in one place or spot than in another. It has not heretofore been found feasible to avoid overheating the tips of the teeth.

The cutting edgesin the bottoms of the grooves or notches are in proximity to the body of the shaped tool, and therefore it has not been found feasible to have these lower cutting edges as 30 hard as the tips. Since the bottom of the groove is close to the heavy body of the hob or other tool, said body' must absorb heat rapidly, necessitating therefore the application of over-heat heretofore; and consequently the tops of the 5 threads, the tips of cutter-teeth, have become overheated, and "injured. In order to bring :the

depressed portions to the'required'point of heat ing, overheat has been applied. But the raised.

v 40 not lose their overheat to the body, but retain work. The delicate part would overheat quickly, 0 and thebody-part was apt to crack before hard-n ening. g

The work could not be put into a cold muflle, as it would take too long to bring the mufile and the muflle-furnace to the required heat.- So the 55 mume of the prior high-speed-steel-hardening art was first brought up toa high heat, and 'the high-speed tools were put right into that high heat, to receive their entire heating directly from the original heat'of the muille.

It was found that if the work was inserted in cold condition and brought slowly up to the heat required for the body of the tool, it necessitated overheating the fine parts.

To reduce the trouble from overheating, the expedient was resorted to of preheating the work, so that the final heating up of the heavy body of the tool would not require too much time, it being hoped not to over-expose the delicate portions of the tool.

The prior processes therefore included a method of stepping-up by one or more preheats, followed by a quick final high heat of 2350 degrees. The preheats ranged from approximately 1500 to 1800 degreesFahrenheit.

In the prior method, the tool was placed in a preheating muflle having a temperature of 1650 degrees. Its injury happened at thehigh heats.

It was found that the work should not be exposed to the final heat for longer than six or seven minutes. That is why preheating was resorted to, because the heavy part must not be i left in-the' overheated muilie longer than was absolutely necessary. It was very diflicult to avoid injuring the edges of the forming tool if exposed as long as six or seven minutes.

The toolswere apt to crack, even with all this long (if the body was'up to proper heat), or else (if the fine edges were heated only for the proper length of time), the body was not heated for a sufiicient period. 'In cooling, whether in air or in liquid, the tool was subject to liability to crack. At each step, the work had to be removed from thesealed muflle, thus exposing it to the air. The

whole mass, the work, the muille, and the furnace,

had so much heat that it would cool very Slowly,

so that the work would anneal (instead of hard ening) if it should be attempted to cool it while decarburization; and the final result was, on that account, an inferior tool, because it was soft on the surface,-and it was soft because it had been robbed-of its carbon or other hardening elements.

' Oncertain tools, such as bits, lathe-tools, etc.,

the soft surfacing or skin was ground away; but, in taps, forming-tools, thread-chasers, hobs, cutters of all kinds, reamers, and any tools of irregular shape, it was impracticable, and in many cases even impossible to grind away those soft skins, without spoiling the form or pitch or shape of the tool.

The tools also suffered because of attempts to avoid decarburization.' These attem'pts'resulted in slight carburizing of the edges. This had the effect of reducing the fusing point of these edges.- Thus at least they became rounded or dulled, and sometimes causedincipient melting. The change in the alloy of the steel would be sufficient alone to reduce its cutting efllciency. When the carbon was lost, its hardness was reduced, and there was other deleterious efiect. The present invention avoids either decarburizing or augmenting the charge ofcarbon in the alloy of the tool.

In attempting to avoid decarburization of these high-speed-steel tools, a carbonous atmosphere was employed, andin many cases the work became over-carburized therefrom.-

There would be a loss of carbon from the fine tool.

Loss of carbon is not incurred from transference as in the prior-art preheating. The tool is not transferred or exposed to air. The controlled atmosphere in I the retort enables the tool to be given the necessary time of heating, and enables the body of the tool to absorb the necessary heat.

All parts of the tool, the fine edges and the body portions; to avoidsudden exposure of the workoxidation are exposed to the same heat, for the necessary time. They are heated uniformly throughout, leaving the metal all over in the same condition; not decarbonized or over-carbonized. The alloy continues the same.

The present invention aims to reduce or eliminate troublesinherentin overheating, and to avoid injury especially to the fine edges and thin to high heat; to economize fuel; to expedite the heating; to avoid the loss of carbon at the heating; to avoid injury from prolonging heat treatment of light tools to the same period as is desirable for heavy tools; to reduce the. length of the heating period; to avoid loss of carbon; to avoid to improve the cooling-hardening process, and render it less expensive; to conduce to rapid cooling of heavy tools, without injury;

and to improve quenching of the. heavy tools in oil,"also without injury: to' avoid the formation of scale on the tools; to secure the retention of the -.original cutting edges on both 'thin' and heavy tools; .to increase the output; and to enable the frail tools to stand up as well as the heavyones.

According to the present invention, the necessity for the stepping-up preheats is eliminated. The steel without exposure to injury is. gradually raised to the proper heat.

Dies, plates and chasers used in cutting or fln- I copper-shot, which absorbs the heat, so that the hob 00013 with suflicientspeed to harden.

The high-speed-steel tool (having. a melting point some hundreds of degrees above the. melting point of ordinary tool-steel), may thus be treated by the novel hardening process, including heating the tool while immersed in a slow cur:

rent of gas, which does not oxidize or carburize the high-speed steel; the heat of the tool being brought up from room temperature steadily and gradually until it reaches about 2000 to 2350 degrees 'F., while still immersed in the current of gas. After this high heat has continued for say twenty to thirty minutes, the heating up of the tool is discontinued and it is cooled-hardened while still in the gaseous envelope, the duration of the cooling being either short or long, according to the result desired. The tool in some cases becomes cool so quickly that it may harden without quenching; and the hardening may take place while the tool is in the container through which (if desired) the current is slowly passing. There may be no current at the cooling stage, in some cases. No scale forms. .There is no loss of carbon.

' Other featuresand advantages will hereinafter appear. 1

In the accompanying drawings,

Figure 1 is a sectional elevation illustrating the improved heating of high-speed-steel tools, and the novel means for accomplishing the results herein set forth.

Figure 2 illustrates the heated work as being rapidly air-cooled, the work, while still enveloped in controlled gas, having been removed from the furnace for that purpose, permitting the heated furnace to be immediately employed for heating up the next batch of cold work.

Figure 2 is a perspective view of the bottom portion of a retort having a work-supporting floor or tray provided with gudgeons set into bayonet-slots in the body of the retort.

Figure 3 illustrates a manner of piling hobs and heavy tools or bodies, and the use of a pyrometer.

Figure 4 illustrates a manner of stacking heavy taps, and other tools, for heating.

Figure 5 shows heavy work withdrawn from the furnace, but still enveloped in the gas, but being more rapidly cooled than by air.

Figures 6 and 7 are sectional elevationsshowing the heating'of heavy tools, and the quenching thereof in oil, without exposing the heated tools to attack from the air.

The high-speed tools III may be piled in cob house fashion, or preferably stood upon a bed ll of loose material which rests upon the bottom I! of a container ii. The work may be so arranged whiie the bottom is out of the container, Then the open lowerend of the container may be set in place-over the work, and attached to said bottom, which is provided with projections II to seat in bayonet-slots IS in the lower end of the container.

Then the container, whose top is closed, is dropped bottom-end first through a large opening IS in the top of the furnace l1, to be heated by flames from the combustion of mixed air and fuel fed by burners IS. The bottom I! of the,

supported upon the bottom 20 of the furnace. The base I9 is of sufficiently small diameter to find room within the slotted lower skirt of the container, and the container bottom may rest upon said base IS. The furnace may be heated by any other means, such as oil or electricity,

H for example.

The top exit of the furnace is closed by a cover 2| A prepared gas is supplied through pipe 23 to the work.

The increase in temperature of the work is container rests upon a base I9, the latter being made graduaL- There need beno steps in the 76 the furnace is never much above the heat of the work, at any moment. Hence the heat, as it is supplied, is allowed to become diffused in the work. There is at no time any serious discrepancy (if any) between the temperature of the fine edges and thin portions of the tools, as compared with the temperature of the massive bodies of those tools. The thintools reach maximum desired heat at the same time that the heaviest reaches the same desired heat.

From the top of the container rises a liftingrod 2|, co-operating with a tackle 22 to lower and raise the container and work; and the rod is made hollow, and connected through a. flexible tube 23 to a source 24 of supply of prepared gas;

provision being made at 25 for reducing or completely shutting off the supply, and at 26 for disconnecting the tube.

The rising in the heat of the work may be continuously observed by a pyrometer 21, whose fire end or element 28 is protectedby a loose tubular jacket 28 pendent from a plate 29, Figure 1, that closes a small opening 30 in the center of the cover 2 l, which may consist of separable segments. The element 28 may be arranged in a central location (Figures 1, 2 and 4) in the group or pile of work-pieces, or extend into central holes in the pieces themselves (Figure 3). The accuracy of the reading of the pyrometer 21 may therefore be depended upon, for the heat from the burners [8 cannot penetrate to the centrally placed element 28 Without imparting to the work-pieces l0, etc., the degree of heat that is exhibited by the pyrometer.

It it should be desired for the base l9 to be attached to the bottom I2 of the canister, it would be lifted out of the furnace therewith, and form a support for the cooling canister. The supply of prepared gas may be detached at 26, and connected to the next charge or its canister, for lowering into the furnace.

The container l3 may be made of highly refractory material, to withstand the high heat aforesaid. It may be an alloy of iron and chrome for example. Or it may be a non-metallic substance, carborundum or other highly resisting material.

Care is taken in practicing the novelmethod to place the cold work in a bath or envelope of gas that is neutral to the tool-alloy, to avoid any chemical reaction, which would be injurious to the tools. The enveloped tools l0, etc. are exposed to heat, whereby they are gradually heated to the desired high point for hardening; and during the heating, the tools remain bathed in the neutral atmosphere, which preferably is in the nature of a current, which flows down through the tool-container l3; the container with the tools being dropped within furnace IT. The gas is preferably trapped in the container,

and the current is so slow that only a little of the 'phere, while another batch, in' another' closedtop container or retort, is substituted in the furnace, avoiding waste of time and heat. The second batch and container are duly removed and replaced by a third batch and container, and so on; each container being placed in unbroken connection with the source 24 of prepared gas, which therefore envelopes the tools during the cooling which is a final step in the hardening process. a

The tool not being exposedto the action of the oxygen in the air, does not check nor surfacespoil in any degree; nor does it have to be surface-ground, but retains its perfect pristine shape, with undulled edges, ready for use. No scale forms; there is no loss of carbon or other elements. From the beginning of the heating until the toolreaches the desired cooled condition, whereby it finally reaches its desired hardened state, it stays right in the container l3, and is continually enveloped in the prepared gas, whereby it is protected from exposure.

It may be noted that exceedingly slow and prolonged cooling of the heated'high-speed steel would not be desired, for it would result in annealing the same.

Examination of the finished high-speed tool under a microscope, reveals uniformity of texture right up to the edge or surface of the tool. No carbon or other element whatever is lost, even from the outer surface. Upon heat-treating a small, thin tool and a quarter-inch drill simultaneously with a heavy square tool about threefourths of an inch thick, in the same container for the same length of time, and enveloped in the same prepared gas, it is found that the small tools and the heavy tool all come out of the treatment in perfect condition, the small ones the same as the heavy one.

In practicing the prior art, a preheated tool may have been so small that it should not be exposed to the high finishing-heat more than approximately a minute or a minute and a half. Largest preheated pieces should not be in the high finishing-heat of the prior art more than six to eight minutes. It was therefore adrawback of the prior art, that the work was not able to withstand high heat for a satisfacory length of time, without becoming subject to in jury, especilly to the fine edges, and to deterioration. in the quality of material.

In practicing the present invention, the work can have a great range in sizes, and the small tools could be in the high heat in the retort together with thelarge tools, for approximately twenty minutes, or even up to thirty minutes, and none of the edges of either the smallipieces or the largest, would be injured; 'and the degree of hardness, and the grain, and the structure, would be found to be substantially the same, throughout the different pieces, small and large.

According to the present invention, the heat is brought up slowly, so that the body reaches the same heat as thethin edges and at-the same time. In practicing the prior art, the heavy body absorbed heat from the thin portions, when the heating was too rapid and the fine edges or projections would be overheated' The present invention overcomes such difiiculties. It is gradually heated, as the metal is able to absorb it.

The duration of the process may be ten or fitteen minutes or -more. It depends upon the weight of the charge.

The great difference between the new invention and the old process, is seen whenthe cold work is put into the retort, which maybe cold, and which goes into the illustrated furnace. The

furnace may be cold, or it may be at 2000 or 2100, 2200, 2300 or 2350. There is no liability of unintentionally overheating the delicate work, 'because the heat has got to go from the furnace 5 through the retort before it can reach the tools, and the heating necessarily can take place only gradually.

The body will absorb the heat from the high points, having sufficient time to do so, on account of the gradual slow application of heat to the retort, that is, the slowly heating body will absorb excess heat that may have reached the points or edges, and hence they will not heat faster than the body, and so will not deteriorate. Where it would normally require three or four minutes for a heavy tool to absorb the heat, any attempt to heat it in only one minute, would cause the thin parts to crack off from the heavy parts. That is why the speed of heating up must not exceed the capacity of the tool to absorb that heat. The heating must keep in balance with the increase in heat.

A fairly heavy tool would take twenty minutes of heating; and a small, thin tool would become heated in five minutes; but they are put into the retort at the same time, the light piece with the heavy piece, and remain twenty minutes, and the small piece does not suffer.

To avoid unequal hardening of the depressed portions and the projecting portions, the degree of heat should be the same and the time of exposure should be the same.

The heat has to penetrate through the slowly heating container or retort, but only as rapidly 86 as the work is able to absorb it. Heat is absorbed in going through that retort and through the gases therein.

The work may be set up or piled around the element-containing pipe in the center of the re- 40 tort. The element 28 may go down inside a hob 3|, Figure 3. The element is therefore at the place which is last reached by the heat which comes in through the retort, and to make sure that the indicated heat has reached the work, 4,; the element is placed in the center. If it is a big tool, the element is placed as near as possible to the center (Figure 1).

No product of combustion has access to the element 28 that is measuring the heat. The heat 50 is measured inside of that retort and right inside of the work, whether it is a hob or a number of .tools.

When the pyrometer 21 indicates the desired high point of heat, it shows that the work has 55 absorbed enough. To make sure, the heat is left on a few minutes longer, so that the heat may diffuse evenly throughout the work, or come to a balance. The hanging element is inserted through the w top of the retort. The connection 23 from the supply of controlling gas may be connected all the time to the retort.

The gaseous atmosphere is thus under control all the time, without changing anything, as the a work proceeds from cold to the required heated condition, and during the cooling. The connec- ""tions are outside the heat. The upper piping 23, etc., is out of the heat. The work and container and element are removed from the furnace.

Upon cooling, the smallest tool is found to be Just as hard as the largest; and vice versa. This is donerwithout damaging the small and frail one.

The tools would not cool fast enough 'if they were left in a muflie-furnace, but would. come out in an annealed state.

Thus by this invention, the high-speed-steel tool, after the described heating, may be hardened without quenching, by cooling it while it 5 remains in the controlled gaseous atmosphere in the retort I3, which may be deposited upon a floorstand for quick cooling, while remaining connected to the prepared gas-supply.

It is not desired to quench in liquid many 10 classes of work. Without using liquid this retort may be taken out and set in a cooling receptacle 32, and shot-copper 33 is poured around it, Figure 5, 'so as to take the heat out of the retort l3 and the work I 0 as fast as desired. Such a 15 quantity of copper cam be employed, as to getthe right hardening. Enough copper is used to reduce the temperature by 1500 or 1000 degrees, in a given time, sufllciently short to cause hardening. If the piece is very heavy, more copper 20 34 must be put in a basin 35 around the retort, so that it will not cool too slowly.

The extraction of heat from heavy tools, by aid of copper, is approximately as rapid as the above-described cooling of light tools in room 25 atmosphere. Heavy tools at Figures 3 and 4, still confined in the retorts, may be thus readily hardened by cooling with copper or its equivalent, in instances where room-cooling would be too slow. The copper absorbs the heat as rapidly 30 from the container and its work, as may be desired; dependent upon the bulk of the copper. Copper is a good heat-conductor, and is as eflicient when surrounding the retort, as if the hot work had been removed from the old muille 35 (after unsealing it) and cooled in the open air; but this invention eliminates the oxidization and deterioration of the old cooling by air-exposure of the work hot from the muille.

If the tool weighs say ten pounds, the cooling will be too prolonged, without artificial cooling means. This is because the heat of the heavy tool itself becomes diffused right around the exterior of the retort, and such surrounding heat must be quickly reduced.

The surrounding heat cannot be taken away promptly if using a prior-art muille.

At Figure 5 is seen the closed retort, surrounded by. the shot. The copper extracts the heat from the retort.

In practicing the present invention, to accelerate the cooling of the work still more, or to quench exceptionally heavy tools, the retort 13, Figures 6, 7, is brought to the surface of a quenching medium (usually oil). Then the 'work-tray 36 is lowered from the retort I3 into the oil bathjl for quenching. All the time the work in the retort is protected by the controlled gaseous atmosphere or envelope, until it is immerse'd in the oil. The work l0, etc., after being 00 heated, is thus cooled apart from the furnace, but without taking it out of the retort, so that the gaseous envelope is notbroken at any time.

When using a muille, there was no satisfactory way of controlling the rate of cooling of the hot tools. They were quenched in oil. They were injured in the air on the way to the oiltank, or in any event the air would attack the tools.

The retort may be brought to orposed over the top surface of the oil, and then may be dropped until the bottom edge of the retort is Just immersed in the oil, coming to rest upon shelf 33, while the tool rest or tray 36 may then drop down into the bath with the work, for 7 quenching, without further immersing the body of the retort, and without waste of gas or exposing the heated work to air.

At Figure '7, the retort is posed right above the surface of the oil, and the tray is allowed to settle from the retortrinto the oil. Thus the work is quenched without access of air thereto.

The work-elevator 39 goes inside of tube 2'] and is lowered into the oil, Figure 7.

The injury arising at the transferring from preheating mufiles, was made worse by transferring the tools from the high-heating stage to the quenching stage, whether cooled in open air or in oil.

It will be observed that in any event (aircooling, copper-cooling or oil-quenching), the high-speed-steel tools are not at any time taken from the controlled gaseous atmosphere in the retort.

The gaseous atmosphere that is put in at the top of the closed retort, no matter what it may be, has to be expelled at the bottom, so that it will be a uniform gaseous atmosphere through- 30 heat that out.

When the tool was taken from the preheating furnace and suddenly put in the old high-heat muffle, its edges were overheated before its body rose to the same degree of heat as those edges. The heat of the overheated mufile itself was the went into the work by prior processes, unevenly heating the fine parts and heavy bodies; but in using this novel process, the heat that goes into the work is the heat that is being applied to the outside of the retort, slowly, and with even heating of all light and heavy parts; the heavy parts having sufficient opportunity to absorb overheat from the fine parts.

In using a mufiie as in the prior methods, the heating-up could be done only for the first batch in any day. At the charging of the second batch, the muiiie would be in red-hot condition, and it could not do the same thing as is done with the cold retort, viz;, the work could not be placed in a cold retort, and then heated up with-the retort.

It was not practicable to put cold work in a cold 45 muflle-furnace, and bring it all up to the highest heat, in an attempt to avoid preheating, for upon removing the hot work from the old mufiie, how could the cycle be repeated? .The muffle-furnace remained hot. Obviously a fresh cold charge 50 could not have been put into that furnace, with the idea of slowly and evenly and uninterruptedly bringing the charge and the furnace to-,

gether up from cold condition to the top heat.

he furnace was already at top heat.

whereas the present invention retains them in the retort, and in five hours the novel cycle may I be repeated ten times; ten new lots. In the 60 same furnace several new charges may be put W high heat.

one after another, without ever cooling the furnace down. It would happen only with the first charge that the cold retort would be heated up with the cold furnace. v

It is not necessary to heat the work to the high pitch of the prior processes, nor is it de- The muffler-user had to take his tools out,

The work being put in the cold retort, and the v carbur-ized or the alloy changed in any way that would affect it detrimentally.

Some high-speed-steel tools may be heathardened by this invention at 2000 or 2100 degrees F. The present invention permits some reduction in heat, as much work can be hardened at 150 to 200 degrees lower. than heretofore, or even at a still lower point. Because the surface is protected, hardening at a lower range of temperatures is rendered feasible.

The highest grade of hardness has been obtained at 2100 by this invention.

The gas may be varied for use with differentalloys. One kind of high-speed steel will absorb more, another less, and the gas must be proportioned or balanced accordingly, as the alloy may require. One has more tungsten, another more molybdenum, another has more nickel, another has more iron, and therefore the gaseous atmosphere has to be varied to suit the purpose, and avoid changing and deteriorating the alloy of the tool.

It is an important feature that there is control of what may occur to the surface of the heating high-speed-steel tools.

The retort may be a chrome-nickel alloy with high heat-resisting qualities. material may be used, for example, graphite, or carborundum. Carborundum can withstand 2500 degrees F. The retort shown is distinguished from an ordinary case-hardening retort, because the latter is not made of refractory material, the heat of case-hardening not exceeding from 1600 to 1700 degrees F.

The gas flowing through the retort for protecting the high-speed steel tools is prepared in the Other refractory form of anhydrous ammonia, which is cracked a before entering the retort in which the work is being treated. This prevents nitriding the highspeed tools. The gas is neutral. If the plain ammonia were introduced into the retort con taining the tools, the heat of the retort would point thereof is not changed. An equilibrium of carbon is maintained between. the gas and the high-speed steel.

At Figure 3 the retort is lifted and deposited by the aid of a cross-bar 40, to the hollow member of which the supply tube 23 is attached. The element 28 extends up to the top of the hollow central rod 2| and emerges from the top thereof. At this view the work-tool s are stood upon a tray 36, which rests upon a disk or plate 4|,

which may rest upon detachable cross-bars 38, set into holes 42 in the retort, and preferably joined in the form of a staple.

At Figures 3, 4 and 6, an additional pyrometer 43 may have an element 44 extending down into the heating chamber of the furnace, to ascertain the heat of the furnace close to the: retort.

At Figures 4 and 5, the upper ends of heavy tools may be set into guide-holes formed in-a disk 45 at the top of the retort chamber, the disk being mounted upon the top of a hollow column 46 which rises from the annular tooling bars 38 may be withdrawn after the bottom of the bell has been set down into the upper portion of the oil, so that there is no access 01' air to the hot tools, and then the tray with its tools may be let down into the oil to quench them, as seen at Figure 7. The tray is lowered and raised by the rod 39, which is provided with a top handle or cross-bar 48. Escape of gas at the top is prevented by a gland at 49, Figure 7.

Variations may be resorted to within the scope of the invention, and portions of the improvements may be used without others.

Having thus described my invention, I claim:

1. A process of hardening which consists of enclosing and fixing work in a bell envelope, to form a hardening unit, inputting a stream of protective gas into the top 01 the bell envelope substantially continuously, to surround said work and venting the gas at the bottom of the en- 'velope, during substantially the entire process,

heating said envelope and thus indirectly heating said enclosed work, removing said unit to a quenching place, quenching said unit, and thereby chilling and hardening said work while fixed in said envelope and substantially surrounded by gas from said stream.

2. The process of hardening work by abrupt chilling, which consists of inclosing work in an envelope provided with a top hole and a bottom hole, introducing a substantially continuous stream oi. protective gas at the top hole and venting same at the bottom. hole and first heating, then quenching the envelope, whereby the enclosed workis hardened while protected by the 88$.

3. A process as in claim 2 wherein the protective gas comprises cracked ammonia.

4. A process as in claim 1 wherein the protective gas comprises hydrogen and nitrogen in substantially the proportions of 3 to 1.

ADOLPH W. MACHLET. 

